1. Field of the Invention
The present invention relates to an imaging apparatus, and, more particularly, to an assembly for limiting axial motion of a shaft in an imaging apparatus.
2. Description of the Related Art
A typical ink jet printer has a reciprocating printhead carriage carrying a printhead in a path transverse to a print media path. A sheet of print media is transported in an indexed manner under the printhead by a feed roller, and the printhead is scanned in a reciprocating manner across the width of an image area on the sheet of print media. The printhead is controlled to expel droplets of ink on the sheet of print media to form an image from a multitude of ink dots. A platen is provided opposite to the printhead for contacting the non-printed side of the print media and, in part, defines the distance between the printhead and the sheet of print media.
In an ink jet printer, dot placement is critical to achieve acceptable print quality. The mechanism that indexes the print media, i.e., the feed roller, must do so with extreme precision. One form of dot placement error is caused by the axial motion of the feed roller, wherein the media position at the beginning of a given swath is displaced from the media position at the beginning of an adjacent swath. This axial motion is a result of indexing system components, for example shafts, which possess unacceptable levels of run out during the indexing operation, resulting in horizontal print registration errors.
What is needed in the art is an apparatus which minimizes the axial motion of a feed roll during the indexing operation, so as to reduce the occurrence of horizontal print registration errors.
The present invention provides an apparatus that minimizes the axial motion of a feed roll during the indexing operation, so as to reduce the occurrence of horizontal print registration errors.
The invention comprises, in one form thereof, an assembly for limiting axial motion of a shaft in an imaging apparatus. Such an imaging apparatus has a frame and a drive system including a helical drive gear. A shaft is provided that is supported by the frame. A driven helical gear is attached to the shaft and positioned to engage the helical drive gear, wherein driving the driven helical gear in a first rotational direction results in an axial thrust of the shaft in a first axial direction. The assembly includes a thrust member and a thrust bracket. The thrust member is provided with an engagement end and a mounting end. The engagement end has a first cross-sectional dimension and the mounting end has a second cross-sectional dimension that is larger than the first cross-sectional dimension. The thrust member is positioned to be co-axial with the shaft and is coupled to at least one of the shaft and the driven helical gear. The thrust bracket is mounted to the frame. The thrust bracket has a contact surface that is engaged by the engagement end of the thrust member when the driven helical gear is driven in the first rotational direction.
An advantage of the present invention is that the axial motion of a shaft in one direction of shaft rotation is reduced to essentially zero as the thrust member contacts the contact surface of the thrust bracket.
Another advantage is that when the invention is applied to an indexed feed roller in an ink jet printer, the amount of horizontal print registration error resulting from an axial motion of the feed roller is reduced to essentially zero.